For Cocoa apps this is easy (I don't know about Carbon ones... never looked into it).
If your App is Cocoa, go into the application bundle and find the "MainMenu.nib" file (it could be named something else, but this is the default name, which most use). Open it in "Interface Builder" which is free with the OS X Developer Tools. From there, you can easily add whatever keyboard shortcuts you like to any menu command you like.